1. Field of the Invention
The present invention relates to a process for preparing a silver halide photographic emulsion having enhanced sensitivity and, more particularly, to a process for preparing a sulfur-sensitized fine grain silver halide photographic emulsion having enhanced sensitivity.
2. Description of the Prior Art
One long pursued object of the art has been to further raise the sensitivity of silver halide photographic emulsions. Another object has been to make the grains in a silver halide photographic emulsion finer in order to improve the graininess or sharpness of photographic light-sensitive materials. However, as grains become finer, sensitivity becomes lower. Thus, it has been particularly desired with fine grain photographic emulsions to raise the sensitivity thereof.
On the other hand, it is of extreme importance to control the fogging of silver halide photographic emulsions. Various compounds have been used for this purpose.
Hydroxytetrazaindene compounds are known as stabilizers for photographic emulsions and have been incorporated in an emulsion in order to reduce the fog thereof. However, these compounds are also known to often reduce photographic sensitivity (see, for example, the paper of V. C. Chambers in Phot. Sci. Eng., Vol. 3, 6, pp. 268 - 271 (1959), entitled A Correlation of the Chemical Structures of Some Triazolopyrimidines with Their Photographic Effects).
We earlier discovered that the sensitivity of a silver halide emulsion can be markedly raised by incorporating a sulfur-containing compound and a hydroxytetrazaindene compound in combination in a monodispersed silver halide emulsion containing cubic grains (silver bromide content: not less than 80 mol%) as we described in Japanese Patent Application 46,398/73.
It is further known that the incorporation of a substituted tetrazaindene compound in a silver halide light-sensitive material containing 20 mol% or more silver chloride causes contrasty sensitization (see Japanese Patent Application (OPI) No. 11,122/74).